Thymus gland assessment in infants and children with atopic dermatitis

Background: Atopic dermatitis (AD) is one of the most common inflammatory skin conditions and is characterized by a significant activation of T cell in lesional and even non-lesional skin. The thymus is a key organ concerned with T cell immune response in early life. We sought to investigate the thymic size in infants and young children with atopic dermatitis and its relation to disease severity. Methods: We conducted a controlled cross-sectional study on a group of 50 preschool children aged 4 years or less with physician-diagnosed AD in comparison to 50 healthy matched children. They underwent thymic index assessment by ultrasonography and complete blood counting with manual differential. Results: Thymic indices of our patients ranged between 0.52 and 34.7 cm3 with a median (IQR) value of 2.7 (2.0 to 9.8) cm3. Relevant values of the control group did not vary statistically (p=0.014) from those of the patients [6.50 (2.40 to 10.80) cm3]. After adjustment for age, sex and weight percentile, there was no statistically significant relation between the thymic index and AD (odds ratio = 1.017, 95% CI = 0.988 to 1.047, p= 0.254). The thymic indices of patients correlated positively and significantly with their oSCORAD indices (p=0.001), and the latter correlated positively with the absolute lymphocyte counts (p= 0.002). Boys had higher frequency of response to treatment of AD as compared to girls (p=0.005). The poor response to treatment was associated with younger age at onset (p=0.003) and high oSCORAD index (p=0.001). Conclusion: Thymic indices were comparable between AD patients and healthy controls, but the thymic size was positively correlated to disease severity. The positive correlation of oSCORAD to thymic size and lymphocyte count reflects the underlying immune dysregulation in AD. Our findings are limited by the sample size and the cross-sectional study design.